Voltage reference circuits are widely used in the electronics art, particularly integrated voltage reference circuits fabricated in semiconductor microchips. A well known technique for obtaining a voltage reference is to utilize the inherent band-gap energy of the semiconductor in or on which an integrated circuit (IC) micro-chip is fabricated. In the case of silicon based IC's, the band gap energy is about 1.1 electron-volts.
FIG. 1 shows typical prior art band-gap voltage reference circuit 9 comprising resistors 10, 16, 28, 29 and 31, capacitor 30, NPN transistors 12, 13, 24, 25, 26, and 32 and PNP transistors 14, 15, 17, 18, 22, 23, and 27, connected as shown. Terminal 33 is coupled to the supply voltage (Vsup), terminal 34 to the reference voltage (Vg), e.g., ground, and terminal 35 provides the reference voltage (Vref) output.
Transistors 22 and 23 form a PNP current mirror. Influence of their base currents is compensated by resistor 16. The influence of the base current of transistor 27 is compensated by the base current of transistor 15. In order to achieve good compensation, resistor 16 must have a relatively precise value. Unfortunately, it is difficult in the manufacture of IC's to maintain a precise resistor value during manufacturing. That is, the statistical variations in the manufacturing process cause the value of resistor 16 in different microchips made in the same production line to vary. This adversely affects the yield and performance of the finished products, e.g., regulators and other circuits which employ reference circuits of the type illustrated in FIG. 1. A further complication is that the value of resistor 16 is usually temperature dependent. This causes; the reference voltage being supplied at output terminal 35 of circuit 9 to vary as the chip temperature varies. This is undesirable. For these and other reasons, there continues to be a need for improved voltage reference circuits and apparatus in which sensitivity to process and temperature variations is reduced.